1. Field of the Invention
The present invention relates to laser processing methods and laser processing apparatus used for cutting objects to be processed such as semiconductor material substrates, piezoelectric material substrates, and glass substrates.
2. Related Background Art
One of laser applications is cutting. A optical cutting process effected by laser is as follows: For embodiment, a part to be cut in an object to be processed such as a semiconductor wafer or glass substrate is irradiated with laser light having a wavelength absorbed by the object, so that melting upon heating proceeds due to the laser light absorption from the surface to rear face of the object to be processed at the part to be cut, whereby the object to be processed is cut. However, this method also melts surroundings of the region to become the cutting part in the surface of the object to be cut. Therefore, in the case where the object to be processed is a semiconductor wafer, semiconductor devices located near the above-mentioned region among those formed in the surface of the semiconductor wafer might melt. In the specification, “wafer shape” means a shape similar to a semiconductor wafer made of silicon of which thickness is about 100 μm, for example, a thin circular shape having a orientation flat therein.
Known as embodiments of methods which can prevent the surface of the object to be processed from melting are laser-based cutting methods disclosed in Japanese Patent Application Laid-Open No. 2000-219528 and Japanese Patent Application Laid-Open No. 2000-15467. In the cutting methods of these publications, the part to be cut in the object to be processed is heated with laser light, and then the object is cooled, so as to generate a thermal shock in the part to be cut in the object, whereby the object is cut.
When the thermal shock generated in the object to be processed is large in the cutting methods of the above-mentioned publications, unnecessary fractures such as those deviating from lines along which the object is intended to be cut or those extending to a part not irradiated with laser may occur. Therefore, these cutting methods cannot achieve precision cutting. When the object to be processed is a semiconductor wafer, a glass substrate formed with a liquid crystal display device, or a glass substrate formed with an electrode pattern in particular, semiconductor chips, liquid crystal display devices, or electrode patterns may be damaged due to the unnecessary fractures. Also, average input energy is so high in these cutting methods that the thermal damage imparted to the semiconductor chip and the like is large.